O-chem 1
Predict the Number of Optically Active Stereoisomers
# stereoisomer = 2^n n= number of chiral centers
If a compound rotates light clockwise
(+) or d (dextrorotary)
If a compound rotates light counterclockwise
(-) or l (levorotary)
Geometric Isomers
*(cis/trans)*: cis = same side; trans = opposite sides.
Alcohol Nomenclature
*Alcohols are named with the -ol suffix* (i.e., butanol, cyclohexanol, 1,3-hexanediol)
Why are ethers desirable solvents?
*All of the above characteristics make ethers excellent solvents* The fact that ethers are unreactive makes them desirable as a solvent because generally we do not want the solvent to react with the reactants or products of our reaction to form byproducts. -The unique polarity of ethers allows them to dissolve many molecules, also desirable for a solvent. -Finally, the low boiling point makes it easier to separate the product from the solvent via evaporation. (longer alkyl groups=higher bp bc more surface area for van der waals forces)
Aromatic Compounds
*Aromatic compounds are conjugated, unsaturated ring systems that exhibit greater stability than one would expect based on either resonance or conjugation alone.* For example, benzene shows far greater stability than other unsaturated compounds with three double bonds and two resonance forms. -Conjugated systems are always very stable, but benzene is far more stable than a conjugated straight-chain alkene.
Oxidation of 3° Alcohols
*CANNOT BE OXIDIZED FURTHER* NO C-H BOND
Determining Hybridization
*Count the number of sigma bonds and add the number of pairs of unbonded electrons.* *remember oxygens lone pairs!!!* -This number will equal the sum of the superscripts on one of the following hybridizations: (Note: No superscript indicates a superscript of one) sp, sp2, sp3, sp3d, sp3d2
Electrophiles
*Electrophiles are electron poor species with a full or partial positive charge* -Electrophiles accept electrons from either nucleophiles or bases.
Homolytic Cleavage
*Forms Radicals*
C6H5
*It represents a benzene*, not an alkane chain. -Also, *there are double bonds in a C6H5 group, so it is unsaturated*. We have seen many students mistake this condensed formula, C6H5 for an alkyl substituent.
Net Dipole Moment
*Molecules with two or more dipole moments can still have no NET dipole when their geometric orientation causes the dipole moments to cancel each other out.* - If two molecules had similar molecular weights and one had dipoles, but no net dipole, and the other had no dipoles at all, they would be expected to have similar boiling points. In other words, a molecule with no net dipole would not have intermolecular forces caused by its individual dipoles. -We have seen many students fail to remember this concept when evaluating a molecule. This concept explains why carbon tetrachloride is non-polar even though carbon and chlorine clearly have different electronegativities. -two same electronegative molecules in opposite directions
sp3
*No p orbitals available for localization*
Amine
1c=1 2c=2 3c=3
Nucleophiles
*Nucleophiles attack carbons* or other central atoms -*Nucleophiles are electron dense and have a full or partial negative charge.* -The best nucleophile is the one that reacts fastest with an available electrophile. -*strong nucleophiles are negatively charged and not bulky* (need to be able to attach to a central atom) -*Nucleophilicity is a function of kinetics—it quantifies the rate at which a molecule reacts with a standardized electrophile -describes how "easily" or "readily" a molecule will react, but nothing about how stable the new bond will be
How to Know Which Mechanism: SN1/SN2 and E1/E2
*REVIEW THE CHART* The major factors that determine which mechanism will be used are: -structure of the electrophile that contains the leaving group, -steric hindrance -the strength of the nucleophile or base.
Which is stronger, a sigma bond or a pi bond? Why?
*Sigma bonds are significantly stronger than pi bonds. This is because sigma bonds allow for electron density to be concentrated to a much larger degree between the two nuclei.* -*The lowest energy state for an electron electrostatically attracted to both nuclei is between those two nuclei and as close to each nucleus as possible.* - In a pi bond the p orbitals overlap above and below the atom, localizing the electrons above and below the plane of the bond—a higher energy state compared to the head-on overlap of a sigma bond. -You can also conceptualize that pi bonds are weaker simply because we know those electrons are in a higher-energy state. -It is universally true that when a bond is higher in energy it will require less energy to break it.
Formation of Alkyl Halide from an Alcohol by SN1 (KNOW THE MECHANISM)
*Tertiary alcohols* STEPS: !) The alcohol acts as a nucleophile, attacking the electrophilic halide hydrogen and forming the good leaving group water plus a halide ion. 2) The good leaving group water spontaneously dissociates (rate-limiting step) leaving a carbocation. 3) The chlorine ion attacks the carbocation to form the product.
Conformational Isomers (Conformers)
*These are not true isomers* -When a molecule twists or rotates around its bonds these are considered "conformers" NOT isomers.
Dehydration of an Alcohol: Synthesis of an Alkene (Know Mechanism)
*This is and equilibrium reaction* *Alkene= favored by hot, concentrated acidic conditions* *Alcohol= favored by cold, dilute acidic conditions* -The major product is the most substituted, most stable Alkene (substituents trans-least steric hindrance) Steps: 1) The alcohol is protonated by the acid 2) The "good leaving group water" leaves, forming a carbocation 3) Methyl or hydride shifts can occur, but only if it results in a more stable carbocation. 4) A water molecule abstracts a proton and the electrons collapse to quench the carbocation and form an alkene. CH3CH2OH + H2O <-> CH3CH2OH2+ <-> CH2=CH2
Combustion of an Alkane
*a radical, exothermic chain reaction* with oxygen; high energy of activation. CH4 + 2O2 -> CO2 + 2H2O + Heat *Heat of combustion is a favorite MCAT topic.*
Formation of Alkyl Halide from an Alcohol by SN2 (KNOW THE MECHANISM)
*primary or secondary alcohols* STEPS: 1) The alcohol acts as a nucleophile, attacking the electrophilic halide hydrogen and forming the good leaving group water plus a halide ion. 2) The halide ion attacks the central carbon via SN2, kicking off water. Don't be confused by the fact that SN2 reactions are usually considered "one step." In this case the protonation of the alcohol is somewhat of a "preparatory" step to transform a hydroxyl group into a good leaving group. This same preparatory step will be necessary in the SN1 reaction described below.
Recognize Common Oxidizing Agents
-*O3* -Cr2O7 -CrO4, -*KMnO4* -*Jones*, -*Collins* -PCC -*PDC*
Label each of the following as electron donating or electron withdrawing: alkyl groups, nitro groups, cyano groups (a.k.a. nitriles), sulphones, amines, carboxylic acids, esters, alcohols and quaternary amines.
-Alkyl groups are weakly electron donating; -nitro groups are strong withdrawing groups; -cyano groups are electron withdrawing; -sulphones are electron withdrawing (assuming connection to the sulfur); -amines are electron donating; -carboxylic acids are electron withdrawing, -esters are electron withdrawing (assuming connection to the carbonyl; if connected to the oxygen of the -OR group, an ester is electron donating); =alcohols are electron donating; -quaternary amines are strongly electron withdrawing.
Predicting Which Bonds are more Polar
-The greater the difference in the electronegativity of two atoms in a bond, the more polar the bond. -HYDROGEN BONDING involves A dipole-dipole interaction where the *H atom is noncovalently attracted to an electronegative atom* -The H must have a large positive delta and the other atom must have a source of electrons to be attracted to H -This shows us that oxygen forms stronger hydrogen bonds than does nitrogen.
Syn Addition vs Anti Addition
-The reaction is an example of *syn addition*, where both new bonds are formed on the same side. In *anti addition*, the new bonds are formed on opposite sides.
Acidity
-increases with positive charge -increases with resonance stabilization - most important!!! -electron withdrawing groups increase acidity (electronegativity increases it-but it decreases with distance away from the atom) -higher s character is more acidic Charge Atom Resonance Dipole Induction Orbitals = *CARDIO*
Newman Projection
-staggered more stable than eclipse -larger substituents separated = more stable (lower energy) -less steric hindrance= more stable (lower energy)
Alcohol Acidity Trends
1) Alcohols are less acidic than water (conjugate base of water has no alkyl groups on it) 2) Alcohol Acidity increases from tertiary to secondary to primary Alcohols where the conjugate base is resonance stabilized will be more acidic. When we look at the conjugate base of water there are no alkyl groups donating electron density to destabilize the oxygen with additional negative charge. With primary, secondary and tertiary alcohols there are one, two and three donating groups respectively. -Therefore, tertiary alcohols are the least stable because the conjugate base in that case is destabilized to the greatest degree by induction *ALKYL GROUPS DESTABILIZE THE CONJUGATE BASE= LESS ACIDIC* Carboxylic acids experience the exact opposite effect because the carbonyl is a strong electron withdrawing group. Further (and even more importantly) the conjugate base in the case of a carboxylic acid is stabilized by resonance. *ELECTRONEGATIVE GROUPS STABILIZE THE CONJUGATE BASE= MORE ACIDIC*
Boiling Point Trends of Alkanes
1) Boiling point increases with increasing chain length and/or molecular weight. (more surface area for london dispersion forces) 2) Boiling point decreases with increased branching (less surface area to form london dispersion forces between molecules) -As the surface area of the molecule decreases (remember that spheres have the lowest surface area/volume ratio of any shape)
IUPAC Rules
1) Find the longest carbon chain. -If there is a tie, the one with the most substituents is the parent chain. 2) The terminal carbon closest to a substituent is numbered #1. -If there is a tie, look at the second substituent. 3) Order the substituents alphabetically and give each one a number to match the carbon to which it is attached. 4) If more than one of the same substituent is present, use the prefixes di, tri, tetra, etc. (i.e., 2,2- dimethylbutane). 5) When to use hyphens: -Hyphens are placed before and after substituent numbers, but NOT between standard prefixes. 6) Rules for alphabetizing: -Do NOT consider the prefix when alphabetizing the substituents if: 1) it represents a number (di-, tri-, etc.) or 2) it includes a hyphen (sec-, tert-, etc.). Do alphabetize other prefixes (isopropyl, isobutyl, etc.).
Exceptions to the Octet Rule
1) Hydrogen and Helium: Stable with only two electrons in their valence shells (e.g., H2) 2) Boron, Aluminum and Beryllium: Stable with only six electrons in their valence shells (e.g., BF3) 3) Atoms from the third period or higher can accept more than eight electrons. Common MCAT examples include: PCl5, SF6, PO43- and SO42-
Melting Point Trends of Alkanes
1) Melting point increases with increasing chain length and/or molecular weight 2) Straight-chain alkanes have the highest melting points. 3) Among branched alkanes, however, increased branching increases melting point. -Going from "branched" to "highly branched" makes a molecule more compact and sphere-like. As the surface area of the molecule decreases (remember that spheres have the lowest surface area/volume ratio of any shape) they will become more compact and thus easier to pack. This explains the melting point phenomenon.
Racemic Mixture
A 50/50 mix of the two absolute configurations of a compound (R and S)
Benzene
A benzene ring is a six-membered ring with alternating double and single bonds. (Note: Of course, the bonds don't really alternate; they form a conjugated pi system often drawn as a circle.) -Three Representations: -Ph, -C6H5 and sometimes -Ar -Don't confuse this: A "phenyl" group (-Ph) is a benzene attached directly to the primary chain; -in a "benzyl" group, a -CH2 is attached to the primary chain on one side and benzene on the other; -an "aryl" group (-Ar) is any aromatic ring system, which would include phenyl groups, but also many others.
Describe the forces responsible for the strength of a bond. What is the relationship between potential energy and bond length?
A chemical bond is a balance of electrostatic forces. We know that species with opposite charges (i.e., nuclei and electrons) will be attracted to each other with a force given by Coulomb's Law: F = Kqq/r2. -However, remember that there are also repulsive forces between the two nuclei and between electrons. -The bond is formed at a sort of "sweet spot" where potential energy is lowest. Increasing bond length or decreasing it would raise the energy of the bond and therefore weaken it.
The Octet Rule
Atoms of low atomic number (<20) tend to gain or lose electrons to obtain exactly eight electrons in their valence shell. -This is the highly stable "noble gas configuration." This is why sodium forms the ion Na+ and not Na2+.
Coordinate Covalent Bonds
A coordinate covalent bond is one in which both electrons shared in the bond are donated by one atom. -Usually more than one of these "donor" molecules (i.e., Lewis bases) surround and bind a single "recipient" molecule (i.e., Lewis acid; usually a metal). -For the MCAT, if a molecule does not have a lone pair of electrons it will NOT participate in a coordinate covalent bond. -the one we've seen most on the MCAT, is easily recognizable. It involves a metal coordinated with one or more molecules that have lone pairs. When you see something like [Fe(NH3)4]2+ or [Co(NH3)6]3+ these structures should stand out because they look almost like ionic compounds, but obviously the amine does not have a charge (so they cannot be ionic). This thought would lead one to wonder how the amines are attached to those metals? The answer is via coordinate covalent bonds. -A common and very applicable example of coordinate covalent bonds in the human body is the binding of iron in the heme unit of hemoglobin:
Optically Active
A substance *does* rotate plane- polarized light.
Hybridization
Atoms, when bonded, hybridize (i.e., mix) their higher and lower energy valence electron orbitals to form "hybrid orbitals" with intermediate energy. -Carbon, for example, has two electrons in the s orbital and two electrons in the p orbital. However, carbon forms four orbitals of equivalent energy when hybridized as sp3.
Axial and Equatorial Chair Conformation
Axial substituents rise vertically from the ring and the equatorial substituents extend horizontally.
Most alcohols are highly soluble in water. Explain why.
Alcohols can hydrogen bond with water, creating a very strong solvent-solute attraction. -The only thing that will reduce alcohol solubility is the presence of long non-polar alkyl chains.
Alkane Suffix for Naming
Alkanes are named with the -ane suffix.
Synthesis of Alkane from Alkene
Alkanes can be formed by reducing an alkene with H2 in the presence of a metal catalyst. CH2=CH2 + H2 /Pd(catalyst) -> CH3CH3 -The reaction is an example of *syn addition*, where both new bonds are formed on the same side. In anti addition, the new bonds are formed on opposite sides.
Oxidation of 2° Alcohols
Always forms a Ketone
Alcohol
An alcohol is any species with an -OH functional group. -Alcohols behave as either nucleophiles (the lone pair on oxygen acts as a Lewis base) -or as Lewis acids when they are oxidized to carbonyl groups (i.e., the oxygen accepts a pair of electrons from the O-H bond as the proton is abstracted).
Aldol Condensation
An aldol condensation is a condensation reaction in organic chemistry in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), followed by dehydration to give a conjugated enone. https://www.khanacademy.org/science/organic-chemistry/ochem-alpha-carbon-chemistry/aldol-condensation-jay/v/aldol-condensation
Alkene Properties
An alkene is any species with a double bond. -*Alkenes are nucleophiles* electron dense! -The pi electrons in the double bond will attack electrophiles forming a new bond to one of the carbons and leaving a carbocation on the other. -The carbocation is then quenched by a nucleophile. -*When located one carbon away from another atom, alkenes are weakly electron withdrawing* -Alkene Stability: *Alkyl substituents (R-groups) increase alkene stability.* Thus, tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted - A tetra-substituted alkene will be the most stable. Because stable molecules have a LOW heat of combustion, the correct answer is the least substituted alkene,
Alkynes
An alkyne is any species with a triple bond. -*Alkynes are almost identical in how they react to alkene*
Polarimeter
An apparatus that *measures the rotation of plane-polarized light as it passes through a sample*.
Ethers Nomenclature
An ether is an oxygen bonded to two -R groups (R-O-R). -Nomenclature: On the MCAT simple ethers are sometimes referred to by common names wherein each -R group is named separately (i.e. methyl ethyl ether, diethylether, methyl propyl ether, etc.). -IUPAC rules call for the standard naming of the longer of the two -R groups as the parent chain, and naming of the other -R group as a substituent with the suffix "-oxy" added. (i.e., 1-ethoxyheptane).
Dipole Moment
Any time charge is not evenly distributed within a bond (i.e., when the two atoms have non-identical electronegativities) that bond will have a dipole moment
Dipole Moment represented on a Molecule
Arrow with a plus sign on its tail. -When this convention is used, the arrow points in the direction to which electrons in the bond will be pulled by the more electronegative atom.
Bases
Bases abstract protons -Bases are electron dense and have a full or partial negative charge -The strongest base is the one that forms the strongest, most stable bond with a hydrogen. -In other words, *for a strong base the acid/base equilibrium favors the conjugate acid, NOT the base* -Basicity is a function of thermodynamics—it quantifies the stability of the reacted base (BH) compared to the unreacted base (B:-) -*basicity describes how much the molecule "wants" to react, but nothing about how quickly it will do so.*
To determine if a group is electron donating or withdrawing:
Electron donating group: -EDG can be recognised by lone pairs on the atom adjacent to the π system, eg: -OCH3 Electron withdrawing group: -EWG can be recognised either by the atom adjacent to the π system having several bonds to more electronegative atoms, or, having a formal +ve or δ +ve charge, eg: -CO2R, -NO2 (ex: carbonyl carbon has partial positive charge) -ALKENES ARE WEAKLY EWG -*Hydrogen is considered neither electron donating nor withdrawing* -*Alkenes are weakly electron withdrawing (just memorize this one*
Basicity
Electron donating groups increase basicity, while electron withdrawing groups decrease basicity. leaving either an OH group or an OR group. The R group is weakly electron donating and therefore feeds extra electron density onto the oxygen, making that oxygen more basic than the oxygen in the hydroxyl group. This is why the alkyl OR- is more basic than OH-.
Crown Ethers
Can interact with different Ions (attraction b/w positive ions and electronegative oxygens -*allow for salvation of Ions in organic solvents* - If there were and ionic interact ex K+F- K+ would be solvated and F- could act as a nucleophile and participate in substitution
What does this tell us about the relative reactivity of the carbons in a C=O vs. a C=N bond?
Comparing electronegativity, we see that the partial positive charge on the carbon in a C=O bond is greater than the partial positive charge on the carbon in a C=N bond. -This indicates that carbonyls are more reactive than comparable imines.
Alkanes
Compounds made entirely of carbon-carbon or carbon-hydrogen single bonds (e.g., gasoline, tar, crude oil, butane, methane, etc.)
Cyclic Compounds Exhibit Ring Strain
Cycloalkanes create ring strain because they force bond angles to deviate from the optimum tetrahedral angle of 109.5°. -*Cyclohexane in its chair conformation has zero ring strain*. -Cycloalkanes with more or less than six carbons exhibit increasing ring strain as one moves away from six -Very large rings (i.e. 10+ carbons) have enough freedom to again approximate the tetrahedral angle. Rings near 10-12 members or above will have ring strain equal to that of cyclohexane. -*Bicyclic ring systems generally exhibit more ring strain than do monocyclic rings*.
Epimers
Diastereomers that differ at only one chiral center. -Many pairs of carboydrates are epimers (e.g., glucose and galactose).
How does polarity relate to nucleophiles and electrophiles?
Polarity results in partial positive and partial negative charges, which are clues when trying to identify the nucleophile and electrophile in a reaction. -To be a base or nucleophile, a molecule must have a full or partial negative charge. -To act as an electrophile, it must have a full or partial positive charge.
Electronegativity
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. -For the MCAT, you should have a general intuition for the relative electronegativity of common atoms. Most of this can come from the periodic table trends covered in the General Chemistry 1 lesson. -It is worth knowing that *carbon and hydrogen have similar, but not identical, electronegativities (carbon = 2.5; hydrogen = 2.2)*. -*Fluorine has the largest electronegativity (4.0)* -*Francium has the smallest (0.7)*. Beyond these general trends, if specific electronegativity figures are required, they will always be given.
Electron Flow
Electrons flow from high concentration to low concentration during reactions. -In other words, they flow from a species with a full or partial negative charge to a species with a full or partial positive charge. -Electron dense species, such as bases and nucleophiles, always attack electron poor species, which are called electrophiles.
Which direction do enantiomers rotate plane-polarized light?
Enantiomers rotate plane-polarized light. *R and S enantiomers rotate this light to the same degree but in opposite directions*. R enantiomers can rotate this light in a clockwise or counterclockwise direction, as can S enantiomers.
Energy is always _______ when a bond is formed. Energy is always ______ when a bond is broken.
Energy is always released when a bond is formed. Energy is always required to break bonds. - One example that often confuses students is ATP. They have the general concept that breaking the phosphate bond in ATP releases energy. That is wrong. Breaking the ATP bond does require energy, but the new bonds formed—in particular the new -OH bond resulting from hydrolysis, releases more energy than was required to break the phosphate bond of ATP.
Epoxides participate in what type of reactions
Epoxide rings can open via either an SN2 mechanism with good nucleophiles, or an "SN1-like" reaction with poor nucleophiles. -In *SN2* reactions the *good nucleophile attacks the least substituted carbon* and -in *SN1*-like reactions the nucleophile attacks the carbocation formed on the *most substituted epoxide carbon.*
Epoxides
Epoxides are cyclic ethers involving one oxygen and two carbons in a three-member ring. -Acid catalyzes the reaction by protonating the oxygen and making it a better leaving group.
Oxidation of 1° Alcohols
From aldehydes and can also form Carboxylic Acids *Do not need to know mechanism, just be familiar*
Oxidation of an Alcohol
General Mechanism
Hoffman Degradation
Primary amide to primary amine
The E/Z Convention
Prioritize the two constituents on each carbon by molecular weight (as you do for R/S). -*If the two higher priority substituents are on the same side = Z*. -If the two higher priority substituents are on opposite sides = E
Reduction Synthesis of Alcohol (Reduce a Carbonyl to an Alcohol)
Generally speaking, reducing agents can reverse the algorithm shown above. o Reducing agents such as -*NaBH4* (one equivalent of hydride ions) -*LiAlH4* and H2/pressure* (2 Equivalents of hydride ions) - extremely strong reducing agent!!!! Metal hydride reagents act as a source of H− because they contain polar metal-hydrogen bonds that place a partial negative charge on hydrogen. reduce a carbonyl to an alcohol *NaBH4 can only reduce aldehydes and ketones* *LiAlH4 and H2/pressure can reduce aldehydes, ketones, carboxylic acids and esters*
Rotation of Plane Polarized Light
Generally, enantiomers have identical physical properties, such as densities, boiling points, melting points, and refractive indices. This poses a problem for experimentalchemists who are working with chiral compounds: how can enantiomerism be observed and measured? Fortunately, there is one physical property in which enantiomers differ: their ability to rotate plane-polarized light. The light that we typically see is unpolarized; that is, it consists of waves that are oriented in every possible direction in an even distribution. We can pass unpolarized light through a polarizing filter to obtain plane-polarized light, which consists of light waves oriented in only a single direction. Solutions of chiral compounds have the property of rotating plane-polarized light passed through them. That is, the angle of the light plane is tilted to the right or to the left after emerging from the sample.
Examples of Electrophiles
H+ R2C=O RX (F, Cl, Br, I) X2 (F, Cl, Br, I) HX R3C+
Absolute Configuration (R and S)
How two enantiomers are distinguished -Assign priority to all four substituents based on molecular weight, with higher molecular weight atoms receiving higher priority (look at first, then second , etc) -Rotate the lowest priority substituent to the back (i.e., into the page). -*If lowest priority is coming out at you, take OPPOSITE LETTER to be its absolute configuration* (would be like taking the mirror image) - If proceeding in order from one to three requires a clockwise motion, the absolute configuration is R; If proceeding in order from one to three requires a counter-clockwise motion, the absolute configuration is S. - These are the simple rules we use when assigning R and S and they are based solely on molecular weight with one important caveat: when comparing identical atoms you look at the atoms attached to each atom with a double or triple bond counting the same as if that atom were bonded to the other atom two or three different times. C=O, for example, is like carbon being bonded to two oxygens and would thus be higher priority than C- OH. Here, the hydroxyl is first because oxygen has a higher molecular weight than the other two carbons. The next in line is the aldehyde group because it has a double bond to its oxygen. Then the methanol group and finally the hydrogen
Shape and Bond Angle are Determined by ____________ and ____________
Hybridization and Lone Pairs
Rule for Aromaticity
Hückel's Rule: To exhibit aromaticity, a ring system must have exactly 4n + 2 pi electrons -A cyclic ring molecule follows Hückel's rule when the number of its π-electrons equals 4n + 2 where "n" is zero or any positive integer, -if an atom is involved in a pi bond, ignore the lone pair. -if two lone pairs are present (not involved in a pi bond) only count one because only one lone pair can align itself with the other p orbital
Grignard Synthesis
Production of an alcohol with extension of the carbon chain - CH3COCH3 + CH3MgBr -> CH3COH(CH3)2 -Produces an alcohol by adding RMgX (most frequently RMgBr; called an organometallic compound) to a carbonyl or other electrophilic double bonds such C=N,cyano groups, S=O and N=O -*Results in an increase in the number of carbons*
Ranking Resonance Structures
If determining the relative contribution of various resonance structures to the actual structure, the individual structure that contributes the most to the actual structure is the one that: 1) Allows the most atoms to have a full octet (unless atom is a common exception to the octet rule) -least amount of charged atoms 2) Has the least formal charge - (i.e., no charge is better than some charge and one formal charge is better than multiple formal charges) 3) Places formal charge on the atom most receptive to that charge (i.e., it is preferable to have a negative formal charge on oxygen than on carbon because oxygen is more electronegative).
Resonance
Important Clarifications: o Resonance structures are a "snapshot" of the different arrangements of electrons that contribute to the "actual structure." o The actual structure is a weighted average (i.e., hybrid) of all of the contributors and does NOT look exactly like any of the individual resonance structures. o Individual resonance structures contribute differently; *the most stable structures contribute the most to the actual structure and the least stable structures contribute the least.* o *The actual structure does NOT resonate back and forth between forms; it is a permanent weighted hybrid of the contributing structures.*
Vicinal
In chemistry vicinal (from Latin vicinus = neighbor), abbreviated vic, describes any two functional groups bonded to two adjacent carbon atoms (i.e., in a 1,2-relationship). 2,3-dibromobutane carries two vicinal bromine vicinal= in the vicinity
Alkoxy
In chemistry, the alkoxy group is an alkyl (carbon and hydrogen chain) group singular bonded to oxygen thus: R-O.
Aliphatic
In organic chemistry, hydrocarbons (compounds composed of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds also known as *non-aromatic compounds* -aliphatics can be cyclic, but only aromatic compounds contain an especially stable ring of atoms
Physical Properties of Alkanes
Insoluble in water; -*very low density* Nearly all alkanes have densities less than 1.0 g/mL and are therefore less dense than water (the density of H2O is 1.00 g/mL at 20°C). These properties explain why oil and grease do not mix with water but rather float on its surface. -non-polar -most are oils or gases
Protection of Alcohols
It is often helpful during a synthetic scheme to be able to protect alcohols from oxidation or related reaction while still allowing the reaction to occur with other functional groups on the molecule. Two common examples are given below: -Protection with TMS: ROH + TMS -> RO-Si(CH3)3 -Protection by MOM (Mothers are very protective! MOM stands for methoxymethyl ether) ROH + Base -> RO- + CH3OCH2Cl (MOMCl) -> ROCH2OCH3 (RO-MOM) Acidification will remove either protecting group to restore the alcohol.
Draw both chair conformations of cyclohexane. In which position, axial or equatorial, would a large substituent be most stable? Try to draw cis-1,2-dichlorocyclohexane with both chlorines in the more stable position (i.e., axial vs. equatorial).
Larger substituents should be in the *equatorial position* because there is more space (more stable, lower energy molecule) -cis-1,2- dichlorocyclohexane cannot exist with both chlorines in the equatorial position. -Two neighboring substituents can be cis to one another, but one must be in the axial position and one in the equatorial position. Below is drawing of the two chair conformations of cyclohexane. *Two neighboring substituents can be cis to one another, but one must be in the axial position and one in the equatorial position* -On these questions you need to draw a chair with axial and equatorial substituents shown. What combinations are possible depends entirely on where they are on the ring compared to each other and whether they are cis/trans; and the various combinations aren't worth memorizing. In this case, when Br and Cl are cis to each other, one must be axial and one equatorial. Bromine should be in the equatorial position because it is the largest -When flipping a chair, substituents switch axial to equatorial and vice vers(but remain either up or down -carbon number rotates in the direction that carbons are counted
Leaving Group
Leaving groups are atoms or molecules that leave the parent molecule during the reaction and *take both electrons from the bond with them*. -*The best leaving groups are those that are the most stable after they leave.* -Good leaving groups are weak bases (ex H20) (Bad leaving groups are strong bases)
Fischer Projection
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Physical Properties of Alcohols (H-bonding, MP, BP)
Like alkanes, boiling point goes up with increasing molecular weight and down with increased branching. Melting point also goes up with increasing polarity and H-bonding Because alcohol substituents can be considered as branching, the effect of branching on the melting points of an ROH is variable. -*Hydrogen bonding is an important concept for the MCAT*. -It explains most of the unique properties and behavior of alcohols and amines. For example, we said earlier that the effect of branching on melting point is less predictable for alcohols. One reason for this is because the major intermolecular force present in alkanes is the Van der Waals force—which is dramatically decreased by branching because it impedes close stacking of the molecules in the solid. In alcohols, however, hydrogen bonding dominates and the impact of Van der Waals forces becomes far less significant. -Hydrogen Bonding: Hydrogen Bond Donors/Acceptors: F, O or N bonded to a hydrogen can act as BOTH a hydrogen bond donor and a hydrogen bond acceptor. -F, O, N atoms with lone pairs that are NOT attached to a hydrogen (i.e., ROR, RCOR, etc.) can act as hydrogen bond acceptors. -*If hydrocarbons have similar molecular weights, alcohols have much higher boiling points and melting points due to hydrogen bonding*
Amide
R= R group or H
Reaction with alcohol and SOCl2 and PBr3
ROH + SOCl2 -> RCl ROH + PBr3 -> RBr
Radiation Radicals
Radicals also form as a result of damage to the molecule itself. -For example, radiation has sufficient energy to strip electrons off of molecules, creating radicals. -That is why radiation exposure is dangerous and carcinogenic.
Use the principles of electron donation and withdrawal to explain why the alpha hydrogens on the methyl side of a methylethyl ketone are more acidic than those on the ethyl side
Looking at the stability of the conjugate base, we see that a carbanion on the alpha carbon of a methyl would experience electron withdrawal due to the carbonyl, but no additional inductive effects. - However, if we examine the ethyl side we see that the same carbanion on that alpha carbon would experience withdrawal due to the carbonyl, but also induction by a weakly donating methyl group (i.e., the beta carbon). This would destabilize the carbanion.
Mesyl Group
Mesyl= methanesulfonyl Ex: Methanesulfonyl bromide
Describe the difference between methyl, primary, secondary, tertiary and quaternary carbons.
Methyl: CH4 -Primary: RCH3 -Secondary: R2CH2 -Tertiary: R3CH -Quaternary: R4C
Anomers
Molecules that differ only in their spatial orientation at the anomeric carbon of a ring structure. -If the anomeric OH/OR group and the CH2OH group are on the *same side of the ring = Beta,* -if they are on the *opposite side = Alpha.*
Meso Compounds
Molecules with two or more chiral centers that contain a plain of symmetry -*This symmetry cancels out their optical activity* -*Look for a center of symmetry & check for mirror chiral centers across the plane of symmetry* -find plain of symmetry and there should be opposite R and S configurations at all pairs of carbons across the plane of symmetry
Examples of Bases
NH2- OH- RO- H:- RC:- R3N H2O NH3
Ammonia
NH3
Examples of Nucleophiles
NH3 *RC=CR* H2O *RMgBr* X- (F, Cl, Br, I) RCO2- CN- Alkenes and Alkynes are NUCLEOPHILES, NOT BASES
Are meSO compounds diastereomers? Are they enantiomers?
No. Diastereomers are PAIRS of compounds that have the same formula, the same bond-to-bond connectivity, are non-identical, and are NOT mirror images. A meso compound is a SINGLE molecule with a mirror plane. Therefore, a meso compound clearly cannot be a diastereomer (or an enantiomer) with itself. *MESO COMPOUNDS ARE ONE MOLECULE*
Chirality
Often referred to as "handedness." -*Any atom attached to four different substituents must be chiral.* -Any atom with less than four substituents cannot be chiral (and therefore cannot be an enantiomer).
Which would have a higher melting point?
One might think that this dipole moment would also increase melting point, but *the importance of stacking to melting point becomes more important.* -Trans- molecules can stack on top of one another closely. However, -cis molecules cannot fit inside of one another and therefore are limited in how closely they can stack. This has been hard to visualize for some students. Consider that the cis- molecules are all the same size. To stack inside of one another they would have to be like Russian dolls—each one smaller than the next. -*The closer stacking of the trans- molecules means that more energy is required to disrupt the Van der Waals forces between them (Mp-cis < Mp-trans).*
Optically Inactive
Optically inactive compounds *do not rotate* plane-polarized light
Single, double, triple bond length, strength
Pi bonds themselves are weaker associations than are sigma bonds. -However, a double bond (one sigma bond plus one pi bond) is stronger together than a single bond (one sigma bond only). -Also, triple bonds are much shorter, stronger bonds than are single bonds, but are also more reactive. -Reactivity has to do with the tendency of the third bond (a pi bond) to react. -Bond strength is a measure of the energy needed to completely break the two atoms apart (i.e., break all three bonds).
Plane-polarized Light
Plane- polarized light is light that exists in only a single plane. -Normally, light waves are oriented in an infinite number of planes from zero to 360°. -A polarizer can sift out all of these leaving, for example, only vertically oriented waves.
E2 Reaction (Elimination)
Rate depends on concentration of two reactants (second order) -One step 1) The single-step abstraction of a proton with collapse of the electrons to form a double bond and ejection of the leaving group. -*strong, negatively charged and/or bulky bases* favor the E2 mechanism - 1°, 2°, and 3° carbons -Favor polar aprotic solvents (cannot form H bonds) because nucleophiles are not hidden by strong interactions with the solvent-more nucleophilic!
SN2 Reaction (Substitution)
Rate depends on concentration of two species (second order) -Involves one step (back side attack) -*Backside attack= inversion of configuration* -The single-step "back side attack" of the electrophile with simultaneous ejection of the leaving group. -*Strong nucleophiles favor SN2 reactions* -*Occurs with methyl, 1° or 2° carbons* -Favor polar aprotic solvents (cannot form H bonds) because nucleophiles are not hidden by strong interactions with the solvent-more nucleophilic!
SN1 Reaction (Substitution)
Rate depends on the concentration of one reactant (first order) -Involves two steps: 1) The dissociation of the leaving group, resulting in formation of a *carbocation* [slow step]. 2) Attack of the carbocation by the nucleophile [fast step]. -*forms a racemic mixture because once the carbocation is formed the nucleophile can attack from EITHER side-only if all three -R groups are different from one another* -methyl or hydride shifts can also occur because a carbocation is formed -*Weak nucleophiles favor SN1 reactions* -*Only occurs with 3° carbons* -*Polar protic solvents (contain O-H or N-H and can for H bonds) solvate well with cations and stabilize the cation intermediate*
E1 Reaction (Elimination)
Rate is dependent on one reactant (first order) -Involvestwosteps: 1) The dissociation of the leaving group, resulting in formation of a carbocation [slow step]. 2) The abstraction of a proton with collapse of the electrons to form a double bond (which quenches the carbocation) [fast step]. -both elimination reactions have *planar* products -favors weaker, neutral bases such as H2O or ROH -*only occurs with 3° carbons* -*Polar protic solvents (contain O-H or N-H and can for H bonds) solvate well with cations and stabilize the cation intermediate*
Rank the bonds listed above according to increasing reactivity.
Reactivity is measured in terms of how readily a species will react in the presence of a nucleophile or electrophile. -*You should definitely know that alkanes are very unreactive*, and that alkenes are far more reactive. -However, it is perhaps surprising that *alkenes are actually slightly MORE reactive than alkynes*. Still, alkynes are far more reactive than alkanes. -The higher reactivity of alkenes is accounted for by the fact that the pi electrons in an alkyne are slightly more stable than the pi electrons in an alkene. -*One potential explanation is that the sp hybridized carbons in the alkyne have more s character and pull more tightly on those pi electrons, making them more centralized and therefore lower energy than the pi electrons in an sp2 hybridized alkene*. Pi bonds are more reactive than sigma bonds BUT alkenes are less stable than alkynes and therefore more reactive
Grignard Synthesis Mechanism
STEPS: 1) Due to the very low electronegativity of Mg, the R group in RMgBr gains significant electron density and more or less acts as if it were a carbanion (R:-), attacking the electrophilic carbonyl carbon. -This occurs in a single step, kicking the electrons in the C=O bond up onto the oxygen. 2) The negatively charged oxygen is protonated, yielding an alcohol.
Structural Isomers
Same formula, different bond -to-bond connectivity Ex: 2-methylpentane and 3-methylpentane are both C6H14
Stereoisomers
Same formula, same bond-to-bond connectivity, but different in the 3-D arrangement of their substituents. There are two categories of stereoisomers: *-enantiomer* *-diastereomers*
Rank the bonds listed above according to increasing bond strength.
Single < Double < Triple
N2 is one of the most stable molecules known; will it have a high or low heat of combustion? Rank cyclopropane, cyclobutane, cyclohexane and cyclooctane according to increasing heat of combustion per -CH2 group.
Stable molecules have very low heats of combustion, so N2 will have a very low heat of combustion. The heat of combustion per -CH2 group increases with ring strain (precisely because more ring strain is less stable). From highest to lowest, heat of combustion is as follows: cyclopropane, cyclobutane, cyclooctane, cyclohexane. Cyclooctane comes before cyclohexane because 8-membered rings are slightly more strained than 6-membered rings. Rings near 10-12 members or above will have ring strain equal to that of cyclohexane.
Cyclic Numbering
Start at one group, make sure next group has lowest number possible
Tosyl Group
Toluensulfonic
Basicity vs. Nucleophilicity
Steric hindrance favors basicity over nucleophilicity; -Nucleophiles must have very little hindrance. -Primary nucleophiles are most common. Secondary atoms can often act as bases or nucleophiles, depending on conditions. However, tertiary atoms will only act as bases. -Reactivity (low stability) favors basicity over nucleophilicity (e.g., NH2- vs. RO-). -If an atom has a full negative charge it will almost always act as a base (halides are a notable exception).
Specific Rotation
Takes these factors into account by dividing observed rotation by the length of the tube and the concentration. -Specific rotation could therefore be described as "observed rotation per length, per concentration unit."
Which would have the higher boiling point?
The -cis version of 2,3-dibromo-2-butene would have a dipole moment and the trans- version would not. -This would increase intermolecular forces and therefore boiling point (Bp-cis > Bp-trans).
IUPAC Nomenclature
The MCAT will almost never require you to provide a correct IUPAC name for a complex organic molecule. -They will, however, refer to molecules in question stems by their IUPAC names, in which case you'll need to recognize those molecules and be able to draw them. -You must also recognize a functional group from its associated suffix. For example, the *"-ol" ending denotes an alcohol*, the *"-oic acid" ending denotes a carboxylic acid*, the *"-oate" ending denotes an ester* and so on
How does the rotation limitation of pi bonds relate to our previous study of proteins?
The amide functional group in a protein (created when the amino group of one amino acid attacks the carboxylic acid group of another amino acid) exhibits resonance and therefore both the C-O and C-N bonds have double-bond character. -This prevents rotation—a key characteristic of peptide bonds.
Which version of 2,3-dibromo-2-butene would have the higher heat of combustion, the cis isomer or the trans isomer?
The cis- version of 2,3-dibromo-2-butene will experience greater steric hindrance, making it less stable, due to the bromine molecules being on the same side of the double bond. -Less stable molecules have higher heats of combustion than do stable molecules, so the cis- version will have a higher heat of combustion than the -trans version.
Coordination Complex
The complex formed by the metal and the molecules forming coordinate covalent bonds with that metal
Observed Rotation
The degree to which a sample rotates plane polarized light. -However, that *rotation is not a universal constant for a particular molecule. Rather, it varies depending on concentration, length of the tube, etc*.
Formal Charge
The difference between the # of electrons in an atom's valence shell when it is in its ground/elemental state and the number assigned (lone-pair electrons and 1/2 of the bonding electrons) to it in a molecule. *Formal Charge = valence - assigned*
The presence or absence of unpaired electrons determines
The exact shape *LONE PAIRS REPEL MORE=LARGER ANGLE*
Space Filling Model
The most conceptually-accurate representation in terms of what you would actually see if you could take an "Incredible Voyage" down to the molecular level would be the space-filling model. -However, remember that those electron clouds would not be nicely color-coded for you. -They are only math functions that predict where the electrons might be. If you were seeing a real methane molecule at that size and scale you would most likely see nothing. -The first thing you could see would be the nucleus and you would need to have magnification many, many times that to see an electron.
Valence
The number of bonds an atom "normally" makes: -carbon and its family are tetravalent; -nitrogen and its family are trivalent; -oxygen and its family are divalent and -fluorine and its family are monovalent. -Count backwards from atom on periodic table
Among similar species that both experience resonance, the more stable specie will be
The one with the most possible resonance structures More resonance structures= more stable!!! (i.e., ClO4- is more stable than ClO3- because perchlorate has four resonance forms and chlorate has only three)
Geminal
The term geminal refers to the relationship between two atoms or functional groups that are *attached to the same atom.* A geminal diol, for example, is a diol (a molecule that has two alcohol functional groups) attached to the same carbon atom, as in methanediol.
Valence Shell Electron Pair Repulsion Theory
The theory that predicts which shape molecules will takedue to the repulsion of lone pairs of electrons. It is often abbreviated as VSEPR.
The Pinac*ol* Rearrangement (Polyhydroxyl Alcohols)- Know the Mechanism
The two hydroxyl groups must be in the vicinal (vic-) position -The carbons bearing those two hydroxyl groups must be tri- or tetra-substituted by -R groups. If tri-substituted, it will yield an aldehyde, and if tetra-substituted it will yield a ketone. -1,2 shift to form more stable carbocation The general rules for carbocation stability can be summarized as follows. (a) Increasing substitution increases stability. CH3+ (methyl; least stable) < RCH2+ (1o) < R2CH+ (2o) < R3C+ (3o; most stable) (b) Resonance is more important than substitution. For example, a secondary carbocation without resonance is generally less stable than a primary carbocation with resonance.
Important Note
There are several species that can be placed in multiple categories. -Hydroxide, for example, will usually be seen acting as a base on the MCAT, but will also act as a nucleophile if there are not acidic protons. -For the MCAT focus on the function: if a species abstracts a proton it is acting as a base and if it attacks a carbon it is acting as a nucleophile.
Predicting Reactions & Products
Things to remember when predicting mechanisms and products: 1) CARBOCATIONS: The mechanism will always proceed through the most stable carbocation (unless peroxide is present). Carbocation Stability = tertiary > secondary > primary 2) STERIC HINDRANCE: If more than one mechanism, intermediate, electrophile, or nucleophile is possible, the one that involves the LEAST steric hindrance will be favored. 3) COUNT YOUR CARBONS: Be sure that the product has the correct number of carbons. Pay special attention to those reactions that add to, or take away from, the length of the carbon chain (e.g., Grignard synthesis, Hofmann degradation, aldol condensation, and acetoacetic ester synthesis.
MCAT Question
This is a classic MCAT question testing a simple principle in a wordy way. Don't freak out, just be very careful and read with precision. Always start by drawing the structures. Whenever we look at acidity, you should have drilled into your head by now, "look at the stability of the conjugate base." In this case, that is the carbanion left behind when the alpha hydrogen is abstracted. Acetate has two sets of 3 alpha hydrogens, one on either side of the carbonyl. If one is removed, we have a carbanion one carbon away from the carbonyl carbon. Because the carbonyl carbon has a partial positive charge, it will withdraw electrons away from the carbanion stabilizing it. This same process occurs to a carbanion formed on acetaldehyde. However, the magnitude of the partial positive charge on the two carbonyls is NOT equal. On the acetate an R group (the other CH3) is weakly donating to the carbonyl carbon making it less positive than the carbonyl carbon of the aldehyde, where only a hydrogen (which neither donates or withdraws) is present. The lower magnitude of the partial positive charge makes it weaker as an electron withdrawing species and thus provides less stabilization to the carbanion. This makes the hydrogens on the aldehyde more acidic. Only answer A describes this phenomenon. Answer A is true, but does not explain the difference between the two acidities. Answer C is false because neither group donates to the carbanion. Answer D is false because both species have an equivalent degree of resonance.
Electrophilicity
This is the easy one—but many students still make errors here. Electrophiles are always electron poor. -They will always have a full or partial positive charge. -Do NOT start a reaction arrow at an electrophile. Electrophiles always get attacked by electron rich species, NOT the other way around!
Formation of Tosylates/Mesylates
Tosyl-Cl + ROH -> Tosyl-OR + HCl -STEPS: 1) The alcohol attacks the tosyl or mesyl halide via SN2, kicking off a halide ion. 2) A hydrogen is abstracted by the halide ion, quenching the charge on the oxygen. *Mesylates and Tosylates are desirable because they make very good leaving groups that will react readily with almost any nucleophile.*
Isomers
Two molecules are isomers if they have the same molecular formula but are actually different compounds. o Isomers are frequently and consistently tested on MCAT-2015.
Relative Configuration
Two molecules have the same relative configuration if their spatial arrangement is identical, but they have *one—and only one—non-identical substituent.*
Diastereomers
Two molecules with the same formula and the same bond-to-bond connectivity that are non-identical, but are -*NOT mirror images* -Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent (related) stereocenters and are not mirror images of each other -There are three kinds of diastereomers you must be familiar with for the MCAT: -geometric isomers, (same name) -epimers (different name) and -anomers.(same name)
Enantiomers (Stereoisomers)
Two molecules with the same molecular formula and the same bond-to-bond connectivity that are non-identical, non-superimposable (not identical) mirror images -They contain at least one chiral center.
Heat of Combustion
When molecules are combusted, all of the bonds are broken and then reformed *via a radical reaction*. The less stable the bond, the greater will be the heat of combustion. The more stable the bond, the lower the heat of combustion - The diagram below shows two RXN coordinate diagrams combined, one for combustion of Reactant1 and one for Reactant2. *Both reactants are hydrocarbons and therefore the products of combustion are the same—carbon dioxide and water.* -That is the key to understanding why less stable molecules release more energy: the energy of the products is always going to be the same (i.e., the energy of CO2 + H2O). -Therefore, the higher the energy of the reactant the greater will be the difference in energy between the reactant and the combustion products. (Note: This diagram shows the energy of the transition state being the same for both reactions, but that is arbitrarily chosen. -This is a path-independent process and so the only concern is the difference in energy between the reactants and the products).
What is the percent s character of the hybrid oxygen orbital in water?
When orbital hybridize, they do so in a weighted manner such that the character of the hybrid orbital is an average of its contributors. Therefore, an sp hybrid orbital has 50% s character and 50% p character. -By this same logic, the oxygen in water, which is sp3 hybridized, exhibits 25% s character and 75% p character.
Rank single, double, triple bonds in stability
When you see the term "stability" on the MCAT they mean "thermodynamic stability"—which in essentially every case will be reflected by the strength of the bond. -Remember that strong bonds are a "lower-energy" state and weaker bonds represent a "higher-energy" state. -Therefore, *triple bonds are the most thermodynamically stable, meaning it will require the most energy to break them apart*. In increasing order of stability they would be ranked: single < double < triple.
Leaving Groups
X ; -OCOR -H2O+ -Tosyl -NH3+ -N2+ (diazonium)
Can an optically inactive compound rotate plane-polarized light?
Yes. A compound with an even number of chiral centers could be said to be rotating light internally, but the net effect of say two R chiral centers plus two S chiral centers is zero net rotation. -*Chiral centers are rotating light but in opposite directions, it cancels out*
Reactions of Ethers
You are unlikely to see an ether participate in a reaction on the MCAT. It will almost always be thesolvent and it therefore should NOT be participating. -If ethers do react, it will only be after the oxygen is protonated by strong acid. The resulting unstable intermediate could then be attacked by a nucleophile.
Remember the following regarding geometric isomers:
a) Cis isomers often have a dipole moment, but trans isomers usually do NOT (i.e., cis- 1,2-dichloroethene vs. trans-1,2-dichloroethene) b) Cis isomers often experience "steric hindrance," resulting in a higher energy molecule, but trans do NOT. In the cis isomer the two polar C-Cl bond dipole moments combine to give an overall molecular dipole, so that there are intermolecular dipole-dipole forces (or Keesom forces), which add to the London dispersion forces and raise the boiling point. -In the trans isomer on the other hand, this does not occur because the two C−Cl bond moments cancel and the molecule has a net zero dipole (it does however have a non-zero quadrupole).
Remember the following regarding enantiomers:
a) They have opposite R/S configurations at every chiral carbon (mirror images). b) They rotate plane-polarized light to the same degree, but in different directions. c) They have all the same physical properties (boiling point, reactivity, etc.) EXCEPT for: -1) how they rotate plane polarized light and -2) *the products they form when reacted with another chiral compound.*
Acetoacetic Ester Synthesis
https://www.youtube.com/watch?v=zpyb8LJWWyA ???
Pi bonds
involve side-to-side overlap of two p orbitals. -Because pi bonds require side-to-side overlap, the atoms must be fairly close to one another. -*As the radius of either atom increases, the p orbitals are spread apart, resulting in less overlap and a weaker pi bond.* This is why a C=N bond is weaker than a C=O bond. -as radius decrease p orbitals become closer together, resulting in shorter bond
Sigma bond
involves directional head-to-head overlap of two atomic orbitals.
Drawing Resonance Hybrids
o Atoms can never be moved. o Single bonds can never be moved. o All structures must obey the octet rule (excluding the exceptions outlined previously). o All structures must have the same number of total valence electrons (i.e., the number of electrons in bonds plus the number of electrons in lone pairs). -This is NOT to say that individual atoms will not change their valence. In one structure they may have a formal charge and in another they may not, but in all structures the total number of valence electrons for the structure as a whole must be constant. o The tail of an arrow showing electron flow during resonance can only start from a lone pair, a double, or a triple bond.
Ether Properties
o Very non-reactive o Weakly polar o With short -R groups ethers are slightly soluble in water o Most non-polar species are soluble in ethers o Low boiling point (no H-bonding)
The second and third bonds are always ______ bonds
pi () bonds
The first bond formed between two elements is always a _________ bond
sigma bond (σ)
Hybridization and Shapes of Molecules
sp=Linear[180 ̊] sp2=Trigonal Planar or Bent[120 ̊] sp3 = Tetrahedral, Trigonal Pyramidal, or Bent [109.5 ̊] sp3d=Trigonal Bipyramidal, Seesaw, T-Shaped, or Linear[90 ̊/120 ̊or180 ̊] sp3d2 = Octahedral, Square Pyramidal, or Square Planar [90 ̊]
Dipole Moment Equation
μ= δd The dipole moment can be calculated using where δ represents charge and d represents distance between charges. (Note: Observe that δ is also the symbol often used to denote partial positive and partial negative charges; i.e.,δ+ and δ-).
